This invention relates to apparatus and methods for testing of materials, and particularly to such apparatus and method for testing the multiaxial impact strengths of materials.
An important criterion in the selection of materials, for example plastics materials, for use in any given application is the resistance of such materials to physical impacts. It is necessary to be able to test plastics materials in as reproducible a way as possible, particularly as a method of control in manufacture. For the latter purpose it is also desirable that testing be carried out quickly so that any tendency for the product to depart from the desired specification can be detected and corrected as soon as possible, and the results should also be reproducible. It is also desirable that the apparatus should be capable of carrying out a series of tests, and recording the results, automatically.
A distinction is to be drawn between uniaxial and multiaxial impact testing, an example of the former being the well-known Izod method wherein a notched bar of the material under test is clamped at one end and struck by a weighted pendulum so that such bars always break in the same direction. In a multiaxial impact test, on the other hand, no attempt is made to induce the direction of failure and the sample breaks in its weakest direction. It is believed that multiaxial impact testing is more representative of the way in which actual articles fail as a result of impacts experienced in use.
The most generally used type of apparatus for multiaxial impact testing involves the use of a dart to which weights are attached and which is allowed to fall from a height on to a sample of the material to be tested. Depending on whether or not the sample breaks as a result of this treatment, weights are either added to or removed from the dart until a weight is found which is just sufficient to break a statistically significant number of samples. Alternatively, the height of fall can be adjusted to achieve the same result. Since this method necessarily involves considerable operator time, not only in carrying out the tests themselves but also in preparing a sufficient number of samples, efforts have been made to devise a test that does not suffer from these disadvantages. As a result of such efforts, testing apparatus has been proposed that would measure the force exerted on the sample by the falling dart by means of an appropriate transducer placed beneath the sample or on the dart, the signals from the transducer being processed by a computer so that the instrument can provide a direct readout of the energy necessary to break that particular sample.
Examples of impact testing apparatus that have been designed in accordance with the foregoing proposals have, however, still suffered from the disadvantages that they consume many samples and are not automatic in operation. They require that an operator position each sample by hand, raise the dart and, after fall, remove the broken sample before repeating the test on another sample. They are noisy and there is also the danger of personal injury to the operator or damage to the apparatus if the weight is allowed to fall prematurely. Because of their size and the need for frequent operator access, it is not practical to enclose such apparatus so as to reduce noise or increase safety. It has been proposed to employ a dart driven by mechanical means rather than accelerated by gravity. According to that proposal, energy stored in a flywheel was applied to a dart through a crank on a shaft coupled to the flywheel by means of a clutch. Only a small portion of the crank's throw was utilizable, because of non-linearity problems, and the apparatus, although representing an advance in the art, was still large and somewhat noisy and did not gain general commercial acceptance. It has also been proposed to use a dart driven by servo-controlled hydraulic means.